1. Field of the Invention
This invention relates in general to radio frequency (RF) shielding for a room and more particularly to coplanar RF door seal utilizing layered flexible metal sheets.
2. Discussion of the Prior Art
It is sometimes necessary to provide a room of relatively large size which is shielded against electromagnetic radiation. Such shielded rooms are used, for example, for circuit testing for RF emission tests, for housing certain computer installations, etc.
A shielded room of the type to which the present invention is directed is a specially made enclosure and is of sufficient size to permit workmen to conduct various work operations therein. The enclosure must be provided with an opening of sufficient size to permit passage of workmen and equipment therethrough. A primary problem with construction of a shielded enclosure is to provide a door which will completely seal the opening against electromagnetic radiation.
The shielded room and door must be constructed of a metal which will afford substantial protection against electromagnetic radiation. In the prior art there have been serious problems associated with the design and construction of shielded doors. The seals have not been effective because of gaps between the seal and the door. The reliability of the seals is poor due to fragile construction and thus will not stand extended usage without frequent repair.
Early prior art shielded doors are usually held against a wall of the shielded enclosure by a latch. In actual practice, the engaging surfaces of the door and wall are not perfectly flat. Thus, to the extent that the surfaces are not completely flat and coplanar, gaps between the surfaces are created. In an effort to overcome this problem, the engaging surfaces of the door and enclosure are meticulously machined at considerable expense. The door must then be very accurately mounted on the wall to assure proper engagement of the surfaces in the closed position of the door. Notwithstanding these expensive and tedious work operations, prior art constructions have proved unsatisfactory in that the latch may hold a local region of the door tightly against the wall, but it does little to assure that the entire periphery of the door will be tightly sealed to the wall surface.
Another attempt to effect a workable seal has been to use integral expander tubes. The prior art doors utilizing these integral expander tubes have been heavy, costly, complex, and subject to considerable maintenance. These doors have been inordinately thick so as to cause excessive vehicular and person traffic problems caused by the inherently large gap in which the door slides. Where the doors are supplied with integral expander tubes, they also must be supplied with compressed air lines to the door and thus require the moving of the air hoses along therewith. The weight of the heavy doors causes considerable difficulty in manual operation that results in safety hazards.
More recently (as in U.S. Pat. No. 4,370,831 issued to Hamilton), an RF shielded door seal has been formed of peripheral flexible plates that are welded together on their outer edges. Spaced between the plates and between the frame and the door, a peripheral tubular bladder is located. The bladder, when expanded with compressed air, forces the plates outwardly against the door and the frame to form a seal. Problems of leakage, however, still persist in this construction due to irregularities in the flexible plates created during the welding process. RF radiation can leak through these small deformities caused by welding the plates together and it would be desirable to have a seal which prevented this problem.